Nondestructive measurements of materials can be performed with a variety of sensors operating on different physical principles, such as electromagnetic, acoustic, or thermal sensors, where the properties of the material influence the response of the sensor. Electrical measurements at the sensor terminal, such as the electrical impedance or admittance, are then used to determine the material properties. These material properties may reflect characteristics of the bulk material condition such as heat treatment, state of cure, fatigue damage, or porosity, surface conditions such as roughness, shot peen intensity, coating thickness, and coating condition, or the presence of defects such as cracks, inclusions, or service-related aging.
In many inspection applications, large surface areas of a material need to be tested. This inspection can be accomplished with a single sensor and a two-dimensional scanner over the material surface. However, use of a single sensor has disadvantages in that the scanning can take an excessively long time and care must be taken when registering the measured values together to form a map or image of the properties. These shortcomings can be overcome by using an array of sensors or an array of elements within a single sensor, as described for example in U.S. Pat. No. 5,793,206, since the material can be scanned in a shorter period of time and the measured responses from each array element are spatially correlated. However, the use of arrays complicates the instrumentation used to determine the response of each array element. For example, in one conventional method, as described for example in U.S. Pat. No. 5,182,513, the response from each element of an array is processed sequentially by using a multiplexer for each element of the array. While this is generally faster than scanning a single sensor element, there is still a significant time delay as the electrical signal settles for each element and there is the potential for signal contamination from previously measured channels. An alternative is to use a separate impedance instrument for each array element. However, this can significantly add to the cost of the system, since the impedance instrumentation tends to be expensive compared to the sensors or array elements.
For nondestructive testing of conducting and/or magnetic materials over wide areas, eddy current sensor arrays may be used. These eddy current sensors excite a conducting winding, the primary, with an electrical current source of a prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks. When scanning over wide areas, these arrays may include several individual sensors, but each sensor must be driven sequentially in order to prevent cross-talk or cross-contamination between the sensing elements. Another approach is to use a single drive winding and an array of sense elements, described for example in U.S. Pat. No. 5,793,206.
There is also a need for methods that make calibration and measurement procedures self-consistent so that the resulting measurements are robust and reproducible to justify implementation in production or field inspection applications. Conventional eddy current “pencil probes” used for inspection of engine components, for example, are often calibrated using crack standards. For example, the signal from a typical crack might be recorded and then the range on the instrument might be set so that the crack response is at 80% of the total scale. Then, a threshold might be set so that some minimum crack size is detectable on the standard. This is a useful method if the crack standard well represents the actual component that is being inspected for cracks. Unfortunately, standards that are flat and contain simulated flaws (e.g., fatigue cracks grown from EDM notches with the notches later broached and then the surface etched to “reveal” the cracks) are generally used to determine the POD (probability of detection) for a given flaw size. This is a useful method only when the component is well represented by the standard. If, for example, the probe scanning the actual component is at a higher lift-off (e.g., proximity of the sensor to the surface is not as close) than it was on the standard when the thresholds were set, then the actual detectable crack size would be larger (and perhaps much larger) than assumed.